There are certain foods, particularly low-acid vegetables (some of which are hereinafter enumerated) which require thermal processing for a long period of time at a relatively high temperature (protracted time-temperature processing parameter) in order to kill microorganisms responsible for food spoilage and toxicity. Clostridium botulinum for example produces its lethal toxin only when it has achieved the vegetative form growing under anaerobic conditions in the canned food and this possibility is prevented by the sterilizing process.
Such prolonged processing can literally ruin the texture (material consistency, integrity and firmness) and color of certain heat-sensitive vegetables such as cauliflower and melons, neither of which is presently sold in the canned form, and with respect to other of the more heat-sensitive vegetables such as sliced squash, such prolonged processing renders their texture so soft and so affects their color as to tend to render them unattractive and unappealing to many consumers. These latter processed products presently are not accepted on a general consumer level and therefore are not canned on any significant commercial scale. The vegetables just named are only part on the entire list falling into the heat sensitive category. By "heat-sensitive" is meant those food products which tend to degrade to the point where their texture, color and/or flavor is impaired when conventionally thermally processed.
From the moment of harvest, food undergoes progressive deterioration and preventative measures are often taken to prolong storage life. Food preservation techniques should retain the nutritional value and prolong the stability of the foods' organoleptic properties. By this is meant those properties or qualities of the foodstuff determinable by use of one or more of the human sense organs, the organoleptic qualities including texture, color, flavor and/or aroma.
Methods of preservation involve the application of scientific and engineering principles to control food deterioraton. Modern processes to achieve food preservation are aimed primarily at controlling the growth of microorganisms. The most important means of controlling these microorganisms include heat, cold, drying, acids, sugar, salt, smoke, and curing. The following discussion will briefly describe each of these processes with the corresponding advantages and disadvantages.
Drying is one of the oldest methods of food preservation known to man. Sun drying of fruits, nuts and grains, meats and vegetables is an important method of food preservation. However, since the natural elements are unpredictable, mechanical dehydration equipment is used to maximize heat transfer into the product and afford greater control of the drying variables. Commerically used dried foods include apples, apricots, figs, prunes, raisins, carrots, potatoes, bananas, eggs, and milk. Most dried foods have excellent shelf life and are reasonably inexpensive and, because of convenience, have widespread use in the food service industry.
By freezing a product and drying it under high vacuum conditions, it is possible to produce many dried foods of superior quality than obtained with conventional drying. Freeze drying is currently used for selected fruits and vegetables, shrimp, coffee, and special military rations. However, they are very expensive, costing much more than conventional dried foods and frozen or canned foods.
A food concentrated to 65% or more soluble solids, (largely composed of sucrose and other sugars) may be preserved by mild heat treatment provided the food is protected from the air. Examples are syrups, jams, jellies, preserves, and sweetened condensed milk. However, due to the high sugar content the preservation of foods by this method is not applicable to most foods in the human diet. Two types of such mild heat treatment are pasteurization which involves a low heat of about 130.degree. F. to 155.degree. F. which alone does not commerically sterilize but must be combined with a food preservative or preservation system, and hot filling which involves filling a container with a food product heated at about 180.degree. F. to 212.degree. F. which can only commercially sterilize certain limited high acid or high sugar foods.
When used in sufficient quantity, salt has a bacteriostatic effect by creating an environment not conducive to bacterial growth. Salt is used, to a very limited extent, for preserving fish and meat, many times with the addition of smoke, to produce a drying effect and impart a desirable flavor. In the amounts necessary to inhibit bacterial growth, salt imparts a harsh, dry, salty taste that is not very palatable, has health implications and is objectionable to many consumers.
Smoke from burning wood contains traces of formaldehyde and other chemicals unfavorable to microorganisms. In addition, smoke is generally associated with a mild heat treatment, usually effected at from about 120.degree. F. to about 160.degree. F. and the resulting dehydration of the food (meat and fish) contributes to its preservation. The resultant product is very dry and not very palatable. Currently, smoke is primarily used for flavor rather than preservation.
Curing is a process which involves chemically treating a meat foodstuff for preservation. While salt is basic to all mixtures for curing meat (bacon, ham, sausage, etc.) sodium nitrate and sodium nitrite have been used as part of the cure for centuries to stabilize the red color and inhibit growth of a number of food poisoning and spoilage organisms. Salts of ascorbic acid and erythorbic acid, and glucono-delta lactone (the lactone hydrolyzes to gluconic acid) are used to hasten development of and to stabilize the red color of red meat. Modern day methods of manufacturing cured meat products include mixing the curing salts with the raw ground meat emulsion (luncheon meat, sausage, etc.) or pumping the curing solution into the raw meat (ham, bacon, etc.) followed by cooking in hot water (150.degree. F.-165.degree. F.) to obtain an internal product temperature between 140.degree. F. to 155.degree. F. These mild cures currently used are not sufficient to produce shelf stable meat products and therefore the meat must be kept under refrigeration.
Microorganism are sensitive to acids in various degrees. The preserving effect of acid is due to the hydrogen ion concentration and its destabilization effect on bacterial cells. Acids may be found in foods as a natural component, produced in foods by fermentation, or added to foods directly as a chemical. Since acid enhances the lethality of heat, acid foods (pH 4.6 or below) need only be heated generally up to about 205.degree. F. which is much lower than the heat needed for more alkaline foods (low acid; pH above 4.6) to render them free of spoilage organisms. The acids commonly added to foods (acetic, citric, malic) create a distinct "pickled" flavor, which in many instances detracts from a natural home-cooked flavor and the foods to which they have been added are technically termed acidified.
Although not a sterilization process, low temperatures (0.degree. F. or below) inhibit bacterial growth and enable forzen foods to be stored for several months with very little deterioration of quality or loss of nutrients. Most meats, fish, vegatables and fruits freeze well and have high organoleptic qualities. It is generally recognized that quickfrozen foods retain the color, texture and flavor of fresh vegetables better than any other food preservation method. However, because of rising energy rates, warehousing, transportation and storage, the cost of frozen foods is substantially higher than for canned food or dried food.
The process of preserving foodstuffs in sealed containers, known as "canning," dates back to 1809. Low-acid foodstuffs (as distinguished from acid foodstuffs which can be hot filled) are filled into a metal container, hermetically sealed and thereafter preserved by thermal processing at a time-temperature parameter sufficient to commercially sterilize the contents. The parameters for such low acid foodstuffs range from about six minutes to about seven hours, and from about 212.degree. F. to about 275.degree. F., the parameter depending upon various factors such as the type of and initial temperature of the product, the size of the container, the type of sterilization process used, the operating parameters of the equipment employed, energy costs and the through-put desired.
Prior to development of canning, foods could not be preserved, transported or stored for long periods except in the dry state. Canned foods were the first "convenience" foods. Today's canned foods are not only convenient and nutritious, but are the least expensive compared to other ready-to-serve foods preserved by freezing or any other means.
To successfully preserve most foods, sufficient heat is required to render the food free of viable microorganisms having public health significance, as well as any other microorganisms of nonhealth significance capable of reproducing in the food under normal storage conditions. The amount of heat and time required to commercially sterilize low acid canned foods usually alters and is sometimes damaging to flavor, texture and color compared to the fresh product. Therefore, any treatment that can be made to the food to reduce the time or temperature necessary for sterilization is desirable since it will generally improve quality. Aseptic canning and rotary agitated cookers are examples. of equipment which provide high temperature-short time commercial sterilization and, therefore, improve quality. Heretofore, in general, commercial sterilization times and temperatures for low acid foods were from ten minutes to six hours at from about 230.degree. F. to 270.degree. F. These times and temperatures are selectable and vary as pointed out above.
The amount of heat necessary to sterilize acid or acidified foods (pH 4.6 or lower) is substantially less than required to sterilize low acid (above pH 4.6) foods. The amount of those acids commonly added to foodstuffs which would be required to acidify foods to pH 4.6 or below, would impart a distinct, sour (vinegary) flavor which would render many products unacceptable.
In spite of all the food preservation processes known to mankind there are some heat-sensitive products, especially certain vegetables, delightful to the taste and quite nutritious in the home-cooked form, which are not available at all as thermally processed products in cans, or as such are available only in a quality which has not obtained general consumer acceptance because of lost texture, degraded color or poor flavor, especially low acid, heat-sensitive vegetables.
In accordance with the present invention, combining a low acid heat sensitive foodstuff with a mixture of an aldonic acid with its lactones, preferably gluconic acid with its lactones, by the addition to a low acid foodstuff of the aldonic acid or an aldonic acid precursor, preferably one of its lactones, to achieve a pH of 4.6 or lower, enables the low acid, heat-sensitive foodstuff to be commercially sterilized or canned to achieve shelf stability while exhibiting flavor, texture, and/or color closer to the natural or fresh product, very similar to the fresh, home-cooked product, and without the typical pickled sharp, pungent, or acid flavor associated with acids commonly used in foods. Moreover, when a foodstuff is sterilized in a metal container internally coated with a suitable enamel, in accordance with the process of this invention, the result is considerably less internal corrosion and iron pick-up from the metal container than experienced with other acids employed in foods. Additionally, the results are much less impairment of the natural flavor and longer acceptable shelf life.
A primary object of the present invention is to enable low acid, heat-sensitive foodstuffs to be thermally processed at a reduced parameter whereby flavor, texture or color (or any combination of these) is closer to the natural or fresh product and is more nearly that of the natural, home-cooked product than heretofore. The dietitian's meaning herein is given to "home-cooked," namely that the foodstuff is cooked until just tenderized, to be more edible, while maintaining near maximum nutrient content.
Another primary object of the present invention is to be able to shift the thermal processing parameter for low acid, heat-sensitive foodstuffs from a high value to a lower value (meaning a lower temperature for the same time or the same temperature for less time, or both lower temperatures and less time) whereat texture and/or color is not drastically altered or impaired, while at the same time the natural flavor is substantially retained, for example by not being masked by the taste of another substance such as the pronounced, sharp taste of strong acids such as vinegar and citric acid, or by avoiding the carmelized flavor resulting from long cooking times.
Another main object of this invention is to provide a method for thermally processing low acid foodstuffs in the presence of an acid with its lactones, preferably an aldonic acid with its lactones in a container, wherein the presence of the acid lowers the equilibrium pH of the contents to 4.6 or below and thereby reduces the severity of processing parameter and improves the flavor of the thermally processed product as compared to that of the same product thermally processed without the acid, and the mildness of the aldonic acid, the level employed and presence of the one or more lactones with the acid softens or modifies the taste of the acid and permits the flavor of the thermally processed contents to be significantly less acid-tasting and significantly improved relative to that of those acids such as for example acetic, citric, lactic, malic, phosphoric and tartaric, commonly employed in foodstuffs.
A related object is to enable any of the foregoing objectives to be accomplished by thermally processing the low acid foodstuff in combination with an equilibrium mixture of gluconic acid and its lactones, glucono-delta lactone and glucono-gamma lactone, the mixture being employed in an amount sufficient to produce an equilibrium pH of 4.6 or lower, where the spores of C. botulinum will not germinate and grow to the toxic vegetative form.
Another advantage and related object of the invention is to achieve a brine (liquid content) which, after thermal processing, is more clear than the brine ordinarily achieved. Since the present invention makes possible reduced processing parameters, another advantage and related object of the invention is that more product substance and nutrients are retained instead of being lost into the brine during processing and storage. Thus, the brine is clearer and the drained weight of the canned product is increased.
Another object of the invention is to enhance the nutrient value of the food product; thus, since the food product has a lower processing parameter, greater amounts of heat-sensitive nutrients are retained in the food.
Another object of the invention is to be able to reduce corrosivity and iron pick-up when thermally processing foodstuff in accordance with the present invention compared to thermal processing of an acidified food when using a conventional acid such as acetic acid (vinegar).